1. Field of the Invention
This invention relates to a device for adjusting exposure in a camera, and more particularly to a small-sized high-performance exposure adjusting device capable of stably activating an aperture member having three exposing apertures with high efficiency.
2. Description of the Prior Art
For exposure adjustment in a still camera or backlight compensation in a video camera, an exposure adjusting device for controlling or partially closing an exposing aperture to dim exposure is incorporated in the cameras. There has been known a conventional exposure adjusting device for continuously varying an exposing aperture by moving two iris diaphragms each having an exposing opening in opposite directions.
In general, a conventional simplified exposure adjusting device used for a simple camera employs one iris diaphragm or aperture plate having two exposing apertures different in size, which is moved rockingly like a pendulum so as to select either one of the exposing apertures. One of the exposing apertures is substantially equal in size to the lens aperture of the camera to form a full aperture, and the other exposing aperture is smaller than the lens aperture so as to reduce the intensity of light passing through the lens aperture.
The iris diaphragm in the simple exposure adjusting device is usually urged by elastically forcing mechanism including a spring or other mechanical elastic means so as to be kept fully open in a steady state, and activated against the elastically forcing mechanism by use of an electromagnetic actuator, solenoid or the like to enter into the optical path in the camera, thereby to partially block off the light passing therethrough.
The aforementioned prior art exposure adjusting device calls for a relatively large force for driving the iris diaphragm because the iris diaphragm is constantly urged by such mechanical elastic forcing means such as a spring. Furthermore, the conventional exposure adjusting device adopting the mechanical elastic forcing means may possibly malfunction due to the current consumption of a power source.
Under the present circumstances in which even a low-priced simple camera has a tendency to be enhanced in performance, a need has been felt for a high-accurate exposing device capable of adjusting the exposure stepwise, e.g. three exposing steps. However, a conventional exposure adjusting device capable of carrying out three-step exposure is complicated in structure and will come expensive, and therefore, it is not suitable for a simple camera moderate in price. Thus, there has been a great need for a simplified high-performance exposure adjusting device for a camera, which is simple in structure and stably operable with a high accuracy.